The alkali-silica reaction (ASR) is a primary cause of premature degradation of concrete structures, particularly for highways, bridgedecks, runways, and sidewalks. Such degradation occurs throughout the world; however, some regions have more frequent occurrences. The reaction occurs between concrete pore water, some aggregate constituents (e.g., poorly crystalline silica phases), and alkali cations (Na and K) released by the cement and/or aggregate. As the alkali content of the pore water increases, dissolution of the silica phases increases, resulting in a release of silica to form the gel. Gel formation results in a volume increase, which causes internal pressure in the concrete and eventually forms fractures. This process can be exacerbated by freeze-thaw cycles and by salting of road surfaces. Thus, ASR products found in concrete may be understood as gelatinous silica, and its crystalline modifications, having adsorbed sodium, potassium and calcium ions.
Important steps in the control of ASR include the recognition of potentially reactive aggregates and early diagnosis of ASR-affected structures. For these and other reasons, it is important to identify the presence of ASR gel in a structure as early as possible. Current approaches to the identification of ASR in concrete rely heavily on petrographic analyses that are expensive and time-consuming (often requiring days or weeks before an answer is obtained).
Recently, the Federal Highways Administration (FWHA) endorsed a staining procedure that was initially proposed in "In Situ Identification Of ASR Products In Concrete" by K. Natesaiyer and K. C. Hover, Cement and Concrete Research 18, 455 (1988) and subsequently tested and refined as part of the Strategic Highways Research Program (SHRP), in "Alkali-Silica Reactivity: An Overview Of Research" by Richard Helmuth et al., SHRP-C-342, National Research Council, Washington, D.C. (1993). This procedure uses uranyl acetate (UO.sub.2).sup.++ (C.sub.4 H.sub.6 O.sub.4).sup.= ! to treat the concrete surface, whereupon the uranyl ion, (UO.sub.2).sup.++, is adsorbed in areas where silica gel is present, replacing previously adsorbed Na.sup.+, K.sup.+ or Ca.sup.++ ions and may be detected by its yellow-green fluorescence under ultraviolet (UV) light.
The uranyl-acetate technique presents several disadvantages. Although the amounts of uranyl acetate employed are small, the technique results in a potential exposure to uranium which is hazardous as both a heavy metal and a radioactive element. Additionally, liquid and solid waste generated by this process is contaminated with uranium, so disposal as mixed waste is costly. Due to the small amounts of uranium used in a typical test, disposal of the materials in the sewer system is attractive. However, this approach unnecessarily adds to system contamination. The use of the uranyl-acetate method also requires a source of UV radiation and a light-tight enclosure for viewing the fluorescence generated by adsorbed uranyl species, making the application of this procedure difficult in a field situation where the use of light-tight enclosures limits the area under investigation, and rendering the observation of a typical road surface tedious and slow.
A method for differentially staining K-feldspar (yellow) and plagioclase feldspar (red) on slab surfaces or uncovered thin sections is described in "Selective Staining of K-Feldspars And Plagioclase On Rock Slabs And Thin Sections," by Edgar H. Bailey and Rollin E. Stevens in Am. Mineral. 45, 1020 (1960). K-feldspar is stained yellow using sodium cobaltinitrite according to the method described in "A Staining Method For the Quantitative Determination Of Certain Rock Minerals," by A. Gabriel and E. P. Cox in Am. Mineral. 14, 290 (1929). The feldspar is first treated by etching the surface to be stained with hydrofluoric acid vapor to form etch residues, since such residues are subsequently stained, whereas the feldspar itself is not visibly stained. Residual potassium from the K-feldspar reacts with the sodium cobaltinitrite to form yellow potassium cobaltinitrite. Staining of plagioclase is accomplished by replacing the calcium ion in the etched feldspar with barium from barium chloride solution, which reacts with the rhodizonate reagent subsequently applied to form red insoluble barium rhodizonate.
Accordingly, it is an object of the present invention to provide a selective process for identifying concrete containing gels formed by the alkali-silica reaction.
Another object of the invention is to provide a selective process for identifying concrete containing gels formed by the alkali-silica reaction that uses environmentally benign chemicals.
Yet another object of the present invention is to provide a selective process for identifying concrete containing gels formed by the alkali-silica reaction that uses environmentally benign chemicals and does not require chemical pretreatment of the concrete.
Still another object of the invention is to provide a selective process for identifying concrete containing gels formed by the alkali-silica reaction that uses environmentally benign chemicals, that does not require chemical pretreatment of the concrete, and that does not require a fluorescence-excitation light source.
A further object of the invention is to provide a selective process for identifying concrete containing calcium-rich gels formed by the alkali-silica reaction.
Yet a further object of the present invention to provide a selective process for identifying degradation of concrete which results in a porous or semi-permeable paste due to carbonation or leaching.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.